Engine with gas exchange through piston rod

ABSTRACT

An internal combustion may include a cylinder having a first combustion chamber at one end and a second combustion chamber at an opposing end, first and second cylinder heads located at an end of the first and second combustion chambers, respectively, and a double-faced piston slidably mounted therein. The piston may be configured to move in a first stroke that includes an expansion stroke portion and a non-expansion stroke portion. The engine may further include first and second piston rod portions extending from opposite faces of the piston. A recess in the piston rod portions may be configured to communicate gases between a combustion chamber and locations outside the cylinder. There may also be a chamber surrounding the first or second piston rod portion, the chamber configured to be supplied with gas and the chamber being isolated from the first combustion chamber and the second combustion chamber.

RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof priority from U.S. patent application Ser. No. 16/588,066, filed Sep.30, 2019, entitled “Engine With Work Stroke And Gas Exchange ThroughPiston Rod” (now patented, U.S. Pat. No. 11,346,219), which is acontinuation of U.S. patent application Ser. No. 15/844,473, filed onDec. 15, 2017, entitled “Engine With Compression And Momentum Stroke”(now patented, U.S. Pat. No. 10,428,655), which is a continuationapplication of U.S. patent application Ser. No. 15/210,596, filed onJul. 14, 2016, entitled “Gas Exchange Through Engine Piston Rod” (nowpatented, U.S. Pat. No. 9,845,680), which itself claims priority basedon U.S. Provisional Application No. 62/192,575, filed on Jul. 15, 2015,entitled “Free Piston Engine,” International Application No.PCT/IL2015/050425, filed on Apr. 22, 2015, and U.S. ProvisionalApplication No. 61/983,469, filed on Apr. 24, 2014, all of which areexpressly incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to the field of internal combustionengines, and more particularly to the field of internal combustionengines having a free piston.

BACKGROUND

Internal combustion engines are known. The most common types of pistonengines are two-stroke engines and four-stroke engines. These types ofengines include a relatively large number of parts, and require numerousauxiliary systems, e.g., lubricant systems, cooling systems, intake andexhaust valve control systems, and the like, for proper functioning.

SUMMARY

Some embodiments of the disclosure may include an internal combustionengine. The engine may include a cylinder having a first combustionchamber at a first end and a second combustion chamber at a second end,a first cylinder head located at an end of the first combustion chamber,a second cylinder head located at an end of the second combustionchamber, and a double-faced piston slidably mounted within the cylinder.The piston may be configured to travel in a first stroke from a firstposition in a region of the first combustion chamber to a secondposition in a region of the second combustion chamber. The first strokemay include a first expansion stroke portion and a first non-expansionstroke portion. The engine may further include first and second pistonrod portions extending from opposite faces of the piston. A recess inthe piston rod portions may be configured to communicate gases between acombustion chamber and locations outside the cylinder. There may also bea chamber surrounding the first or second piston rod portion, thechamber configured to be supplied with gas and the chamber beingisolated from the first combustion chamber and the second combustionchamber.

The forgoing generally describes just a few exemplary aspects of thedisclosure. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only, and are not restrictive of the invention, as may beclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a free piston engine according to thepresent disclosure;

FIG. 2 is a partial cross-sectional view of the engine of FIG. 1 withthe piston at a combustion point on a left side of the cylinder;

FIG. 3 is a partial cross-sectional view of the engine of FIG. 1 withthe piston in a momentum portion of the stroke, in an early stage ofcompressing gasses on the right side of the engine;

FIG. 4 is a partial cross-sectional view of the engine of FIG. 1 ascompression continues on a right side of the cylinder beyond thecompression illustrated in FIG. 3;

FIG. 5 is a partial cross-sectional view of the engine of FIG. 1 in anadvanced stage of compression on the right side of the cylinder beyondthe compression illustrated in FIG. 4;

FIG. 6 is a partial cross-sectional view of the engine of FIG. 1 in aneven more advanced stage of compression on the right side of thecylinder beyond the compression illustrated in FIG. 5;

FIG. 7 is a partial cross-sectional view of the engine of FIG. 1 withthe piston at a combustion point on a right side of the cylinder;

FIG. 8 is a partial cross-sectional view of the engine of FIG. 1 withthe piston in a momentum portion of the stroke, in an early stage ofcompressing gasses on the left side of the cylinder;

FIG. 9 is a partial cross-sectional view of the engine of FIG. 1 ascompression continues on a left side of the cylinder beyond thecompression illustrated in FIG. 8;

FIG. 10 is a partial cross-sectional view of the engine of FIG. 1 in anadvanced stage of compression on the left side of the cylinder beyondthe compression illustrated in FIG. 9;

FIG. 11 is a partial cross-sectional view of the engine of FIG. 1 in aneven more advanced stage of compression on the left side of the cylinderbeyond the compression illustrated in FIG. 10;

FIG. 12, similar to FIG. 2, illustrates a combustion point position onthe left side of the cylinder;

FIG. 13 is a perspective view of a piston assembly that may be used withthe engine of FIGS. 1 and 2;

FIG. 14 is a perspective view of a piston center disk of the pistonassembly of FIG. 13;

FIG. 15 is a perspective view of a left-side piston disk of the pistonassembly of FIG. 13;

FIG. 16 is a perspective view of a right-side piston disk of the pistonassembly of FIG. 13;

FIG. 17 is a perspective view of a piston ring that may be used with thepiston assembly of FIG. 13;

FIG. 18 is a side view of the piston ring of FIG. 17;

FIG. 19 is a plan view of the piston ring of FIG. 17;

FIG. 20 is a perspective view of the piston assembly of FIG. 13 with thepiston ring of FIG. 17;

FIG. 21 is a side view of the piston assembly and piston ring of FIG. 20assembled on the piston rods of FIG. 2;

FIG. 22 is another perspective view of the piston assembly and pistonring of FIG. 20 assembled on the piston rods of FIG. 2 with differentinlet passageways; and

FIG. 23 is a perspective partial cross-sectional view of the engine ofFIG. 1.

DETAILED DESCRIPTION

The present disclosure relates to internal combustion engines. While thepresent disclosure provides examples of free piston engines, it shouldbe noted that aspects of the disclosure in their broadest sense, are notlimited to a free piston engine. Rather, it is contemplated that theforgoing principles may be applied to other internal combustion enginesas well.

An internal combustion engine in accordance with the present disclosuremay include an engine block. The term “engine block,” also usedsynonymously with the term “cylinder block,” may include an integratedstructure that includes at least one cylinder housing a piston. In thecase of a free piston engine block, the engine block may include asingle cylinder, or it may include multiple cylinders.

In accordance with the present disclosure, a cylinder may define atleast one combustion chamber in the engine block. In some internalcombustion engines in accordance with the present disclosure, acombustion chamber may be located on a single side of a cylinder withinan engine block. In other internal combustion engines in accordance withthe present disclosure, the internal combustion engine may include twocombustion chambers, one on each side of a cylinder within an engineblock.

Embodiments of the present disclosure may further include a piston inthe cylinder. In accordance with some embodiments of the disclosure usedin a free piston engine, the piston may include two heads on oppositesides thereof. In some embodiments of the disclosure, the piston may beconsidered to be “slideably mounted” in the cylinder. This refers to thefact that the piston slides through the cylinder from one side of thecylinder to the other. While the present disclosure describes pistonexamples, the invention in its broadest sense is not limited to aparticular piston configuration or construction.

FIGS. 1 and 2 illustrate an exemplary embodiment of a free piston engine10 according to the present disclosure. The free piston engine 10, whichis sometimes referred to herein simply as an engine, is one example ofan internal combustion engine including an engine block 8. A cylinder 12defining at least one combustion chamber may be included in the engineblock and may have a central, longitudinal axis A, and a double-facedpiston 50 reciprocally mounted in the cylinder 12. The double-facedpiston 50 may be configured to travel in a first stroke from a first endof the cylinder to an opposite second end of the cylinder, and in asecond stroke from the second end of the cylinder back to the first endof the cylinder. The first or second stroke may include a work stroke.For example, when the double-faced piston 50 is adjacent to a cylinderhead 14, an injector 34 may inject fuel into a combustion chamber, andan air-fuel mixture in the combustion chamber may be ignited, thuscreating an explosion that drives a work stroke of the free pistonengine 10. The distance of a stroke may refer to a linear distance thatpiston 50 travels along axis A. A width of piston 50 may refer to adistance from a first face to an opposing second face of piston 50 alongaxis A. A width of an exhaust port 18 may refer to a distance from afirst edge of exhaust port 18 to a second edge at an opposing side ofexhaust port 18 in the direction of axis A. FIGS. 7-12 illustrate anexemplary movement of the piston 50 from a first end of the cylinder toa second end of the cylinder. At least one piston rod portion may beconnected to the piston rod and may extend from a location within the atleast one combustion chamber to an area external to the cylinder. Asused herein, the term piston rod portion includes any portion of a rodor shaft, extending from a piston. In some embodiments, a piston rodportion may be a portion of a unified structure passing all the waythrough the piston. In other embodiments, a piston rod portion may be aportion of a piston rod that extends from only one face of a piston.

By way of example, in FIG. 3, a piston rod portion 42 may be connectedone face of the piston 50 and extend from a location within the at leastone combustion chamber to an area 45 external to the cylinder.Similarly, a second piston rod portion 43 may extend from an oppositeface of double sided piston 50, to another area 47 external to thecylinder 12. Piston rod portions 42 and 43 may be integral with eachother, or may be completely separate structures, each extending from anopposite side of piston 50.

An area external (e.g. areas 45 and 47) to the cylinder may include aninlet manifold at each end of the cylinder configured for supplyingcombustion gases to each of the combustion chambers at the opposite endsof the cylinder from one or more sources of the gases external to thecylinder, or an exhaust manifold configured for receiving combustiongases from the combustion chambers and directing the combustion gasesaway from the cylinder for exhaust aftertreatment. In this way, forexample, a passageway of the piston rod portion is configured tointroduce combustion gas into a combustion chamber from a locationoutside the cylinder. In one embodiment, the areas 45 and 47 external tothe cylinder may simply refer to any region on an opposite side of acylinder head 14, 15 from the cylinder 12, regardless of whether theregion is in direct contact with a cylinder head. It is contemplatedthat ports could be provide to introduce gases from a manifold or othersource located alongside the cylinder, rather than at ends of thecylinder. Thus, in a general sense, locations outside the cylinder maybe at either the ends of the cylinder, alongside the cylinder, or acombination of both.

In accordance with embodiments of the disclosure, each piston rodportion may include at least one recess forming a passageway configuredto communicate gas flow between the at least one combustion chamber andthe area external to the cylinder. As used herein, the term “recess” canbe defined by any structure or void capable of communicating gas flow.It may include, for example, a channel or conduit completely orpartially contained within at least part of the piston rod portion. Or,the recess may include one or more exposed grooves or other cut-outs inat least part of the piston rod portion.

For example, in some exemplary embodiments of an engine according tothis disclosure, the one or more recesses forming passageways in thepiston rod portions may render the piston rod portions 42 and 43 atleast partially hollow. In some variations, a passageway may include agroove or grooves formed along an external periphery of the piston rodportion. Still further variations may include different outer diametersections of the piston rod portions. Such area(s) of reduced diametermay provide one or more gaps through which gas may flow. Alternatively,the one or more recesses forming the passageways may include a channelextending internal to a piston rod portion. In yet a furtheralternative, the recess may render the piston rod portion hollow in someareas and partially hollow (e.g., via external groove, slot, etc.) inother areas. At least one port may be formed in each piston rod, influid communication with the passageway of the piston rod portion, tothereby permit gas to enter and/or exit the passageway through the port.

By way of example with reference to FIG. 22, each piston rod portion 42and 43 may include a recess 53, 55, respectively (e.g., hollowed outinternal portion of piston rod portions 42 and 43), forming a passagewayor channel configured to communicate gas flow between the combustionchambers 49 and 51 (see FIGS. 5 and 10, respectively) and respectiveareas 45 and 47 external to the cylinder 12. The hollowed out regionmay, for example, be a bore through a core of a piston rod portion.

As illustrated in FIG. 5, a first combustion chamber may be defined inregion 49, between a face of piston 50 and a first head 14 of cylinder12. Likewise, as illustrated in FIG. 10, a second combustion chamber 51may be defined between an opposing face of piston 50 and an opposinghead 15 of cylinder 12. Of course, it is to be understood that eachcombustion chamber is a variable region that essentially includes aswept volume on each side of the piston, and which is compressed as thepiston moves from one end of the cylinder to the opposite end of thecylinder.

The passageways or recesses 53 and 55, as illustrated in FIG. 22, areexemplary only. For example, as illustrated, the recesses extend justpast ports 44, terminating before reaching piston 50. Numerous otherconfigurations are contemplated within this disclosure. For example,recesses 53 and 55 could extend further toward the piston, all the wayto the piston, or may cross one face of the piston. In a preferredembodiment, passageways 53 and 55 are not in flow-communication witheach other.

In one exemplary embodiment shown in the figures, one or more ports 44,which may be arranged in two groups, i.e., an inner group 46, that isclosest to the piston 50, and an outer group 48, that is distal to theinner group 46. Ports 44 may be configured to serve as inlets forconveying gas into the cylinder via recesses 53 and 55. In lieu of twogroups of inlet ports, only one group of inlet ports 44 may be employed,or more than two groups of inlet ports 44 spaced along the piston rodportions 42 may be employed. Moreover, the inlet ports do notnecessarily need to be arranged in groups, so long as there issufficient opening to convey gases from the channels within the pistonrods, defined by recesses 53 and 55.

In accordance with some embodiments of the disclosure, a firstpassageway and the second passageway in the piston rod portions may beconfigured to prevent gases from being exchanged between the cylinderand a location outside the cylinder via a path that crosses the firstface and the second face of the piston. For example, the pair of pistonrod portions 42 and 43 extending from opposite faces of the double-facedpiston 50 may be integrally formed, or may be indirectly connected toeach other through the double-faced piston. However, no interconnectingflow passageway may be provided between the piston rods. In such aconstruction, no communication of gas flow may occur between thecylinder and a location outside the cylinder that crosses both the firstand second faces of the double-faced piston 50. Thus, the recessesand/or passageways in each piston rod portion may be separate from eachother and may extend through different piston rod portions.

If the cylinder head on each side of the engine block includes (e.g., isconnected to or is integrally formed with) an intake manifold, thepassageway in the first piston rod portion may be configured tocommunicate gas flow between the first combustion chamber and the intakemanifold at the first end of the cylinder, and the passageway in thesecond piston rod portion may be configured to communicate gas flowbetween the second combustion chamber and the intake manifold at thesecond end of the cylinder. Thus, for example, with reference to FIG.10, gases from combustion gas inlet chamber 32 of intake manifold 26 mayenter the combustion chamber as ports 46 and 48 bridge the cylinder head14.

A cylinder in accordance with embodiments of the disclosure may beclosed at both ends. For example, the cylinder 12 of engine 10 may beclosed at both ends thereof by cylinder heads 14 and 15, which may beconnected to the cylinder 12 by a plurality of bolts 16. As used herein,the term “closed” does not require complete closure. For example,despite that the cylinder heads may have openings therein through whichpiston rod portions 42 and 43 pass, the cylinder heads are stillconsidered “closed” within the meaning of this disclosure.

A peripheral portion of the cylinder 12 may be provided with coolingfins 24. Alternative configurations of the engine 10 may include otherexternal or internal features that assist with the cooling of thecylinder, such as water passageways formed internally within thecylinder walls or jacketing at least portions of the cylinder walls forwater cooling, and other configurations of cooling fins or otherconductive and/or convective heat transfer enhancement featurespositioned along the exterior of a cylinder peripheral wall tofacilitate fluid cooling of the cylinder.

Also in accordance with exemplary embodiments of the disclosure, aperipheral wall of the cylinder between the first and second ends mayinclude at least one exhaust port. By way of example only, the cylinder12 may include at least one exhaust port 18 in a peripheral side wall ofthe cylinder 12 between the first and second ends of the cylinder. Inthe exemplary embodiment illustrated in FIGS. 2-12, a plurality ofdistributed exhaust ports 18 may be spaced about the circumference ofthe cylinder at approximately a midpoint of the cylinder 12 between theopposite ends of the cylinder. The exhaust ports 18 may be of anysuitable size, shape, and distribution so as to accomplish the functionof exhausting gases from the cylinder. One of more of the exhaust portsmay, for example, be located in an axial central region of the cylinderperipheral wall, as illustrated in the figures. Although the exemplaryembodiment shown in the figures is configured symmetrically, with theexhaust ports 18 located midway between the opposite ends of thecylinder, alternative embodiments may position the exhaust ports at oneor more radial planes intersecting the cylinder peripheral wall atlocations other than the exact midway point between the cylinder heads14.

In accordance with some exemplary embodiments of the disclosure, atleast one port may be configured to communicate gas flow between thefirst combustion chamber and outside the cylinder when the piston is onthe second combustion chamber side of the at least one port, and may beconfigured to communicate gas flow between the second combustion chamberand outside the cylinder when the piston is on the first combustionchamber side of the at least one port. By way of example only, this canoccur when, as illustrated in FIG. 5, piston 50 is located to the rightof ports 18, enabling conveyance of gas flow through port 18, from thecombustion chamber to the left of the piston 50. Ports 18 enable gasflow to a location “outside” the combustion chamber. That outsidelocation may be on the side of the cylinder as illustrated, or conduits(not shown) associated with the engine might deliver the gases to otherlocations.

The inlet manifold 26 may be connected to or formed integrally with eachof the cylinder heads 14, 15 at opposite ends of the cylinder 12. Theinlet manifold 26 may include a piston rod opening 28 that is axiallyaligned with the longitudinal axis A, and one or more inlet openings 30,which may be positioned at a distal end of the inlet manifold, as shown,or at any location along the outer periphery of the inlet manifold. Theone or more inlet openings 30 in inlet manifold 26 may be configured todirect inlet gases into the inlet manifold transversely to thelongitudinal axis A. An inner space of the inlet manifold 26 may definean inlet chamber 32. Although the inlet manifold of the exemplaryembodiment shown in FIGS. 1-12 and 23 is illustrated as having acylindrically-shaped configuration, alternative embodiments may provideone or more inlet manifolds with other shaped profiles or crosssections, or may incorporate the inlet manifolds at least partiallywithin the cylinder heads 14, 15 as one or more internal passagewaysdefined within each of the cylinder heads at each end of the cylinder12.

Each of the cylinder heads 14, 15 may further include one or moreinjectors 34 that open into an annular or toroidal-shaped recess 36formed in or contiguous with a flame face of a fire deck of eachcylinder head at each end of the cylinder 12 in facing relationship withthe combustion chambers at each end of the cylinder 12. Toroidal-shapedrecess 36 may impart swirl flow to fuel gas injected by injectors 34 tofacilitate more complete combustion of the gases within the combustionchambers. The cylinder heads 14, 15 may also include one or morecavities for accommodating and mounting one or more spark plugs 38, andbushings 40 for aligning, supporting, guiding, and sealing (by means ofa dedicated seal) a piston rod portion 42, 43 that is supported by, andpasses through each of the cylinder heads 14, 15 at opposite ends of thecylinder 12. This is one example of how piston rod portions may extendfrom faces of a double-faced piston through a combustion chamber.Regardless of the particular details of any aperture through which thepiston rods may extend at ends of the cylinder, a piston rod thatextends to at least an end of the cylinder is said to extend through acombustion chamber within the meaning of this disclosure.

A double-faced piston consistent with embodiments of the disclosure, maybe configured to travel in a first stroke from a first end of thecylinder to an opposite second end of the cylinder, and in a secondstroke from the second end of the cylinder back to the first end. Thislength of travel is illustrated, by way of example, in FIGS. 2-7, whereFIG. 2 represents an end of a first stroke, FIG. 7 represents an end ofa second stroke, and FIGS. 3-6 represent exemplary intermediatepositions.

According to various exemplary embodiments of the present disclosure,the piston may be sized relative to the cylinder to enable an expansionstroke portion of each stroke wherein the piston travels under gasexpansion pressure, and a momentum stroke portion of each stroke for theremainder of the stroke following the expansion stroke portion. Theexpansion stroke portion of each of the first and second strokes of thepiston is the portion of travel when the piston directly moves under theexpansion pressure of combustion. For example, the expansion portion ofa stroke may be defined as the portion from a position of the pistonwhere combustion occurs (e.g., a combustion point) at each end of thecylinder to the point at which combustion gases may be exchanged betweenthe combustion chamber in which ignition of combustion gases has justoccurred and an area external to the cylinder. The expansion portion ofthe stroke may correspond to the portion of the stroke in whichexpansion gases are trapped in the cylinder.

At the combustion point of the piston during each stroke, a clearancevolume remains between each of the opposite faces of the double-facedpiston and a respective end of the cylinder as closed off by thecylinder heads 14, 15. The combustion gases that have been introducedinto the combustion chamber before the piston reaches the combustionpoint are compressed into the remaining clearance volume on that side ofthe piston between the piston face and the fire deck of the cylinderhead. The compressed gases, which usually include a fuel/air mixture,may be ignited by either a spark, or by self-ignition resulting at leastin part from the compression of the combustion gases. The expansionstroke portion of each stroke occurs after the ignition of thecompressed combustion gases as chemical energy from the combustion ineach combustion chamber is converted into mechanical power of thepiston. Useful work may be extracted from the piston as it moves in theexpansion stroke. Simultaneously with the expansion stroke portion ofeach stroke on one side of the piston, gas flow may occur forsubstantially the entire expansion stroke portion between the combustionchamber on the opposite side of the piston and the intake manifold atthe opposite end of the cylinder, as well as the exhaust manifold 20located at a central peripheral portion of the cylinder.

At the beginning of an expansion stroke portion of a stroke from theleft end of the cylinder to the right end, as shown in FIG. 2, gas flowmay occur between the combustion chamber on the right side of the pistonand the inlet manifold 26 on the right side of the cylinder, and betweenthe combustion chamber on the right side of the piston and the exhaustmanifold 20 through the exhaust ports 18. The communication of gasesbetween the combustion chamber on the right side of the piston and theexhaust manifold may continue until the right face of the piston hasmoved past the centrally located exhaust ports 18, acting as an exhaustvalve and shutting off communication between the right combustionchamber and the exhaust manifold. Additionally, before the piston 50 haseven closed off the exhaust ports 18, the inlet ports 44 closest to theright face of the piston may have moved outside of the right combustionchamber, thereby closing off communication of gases between the rightinlet manifold 26 and the right combustion chamber through the rightpiston rod portion 42.

According to some embodiments, a length of the double-faced piston, alength of the cylinder, a location of the exhaust outlet, and a locationof a channel access opening in each of the first and second piston rodportions may be arranged such that when the piston is in a combustionstage in the first combustion chamber, the piston blocks the exhaustoutlet from communicating with the first combustion chamber and thechannel access opening in the first piston rod portion is outside of thefirst combustion chamber, while simultaneously the exhaust outlet is influid communication with the second combustion chamber, and the accessopening of the second channel is within the second combustion chamber.This may be accomplished by various structures. By way of example onlywith reference to the figures, the length of the double-faced piston 50,the length of the cylinder 12, the location of the exhaust outlets 18,and the location of the inlet ports 44 in each of the first and secondpiston rod portions 42, 43 extending from opposite faces of the piston50 may be arranged such that when the piston is in a combustion stage ina first combustion chamber on one side of the piston, the piston blocksthe exhaust outlet from communicating with the first combustion chamber.The closest inlet port 44 to the one side of the piston remains outsideof the first combustion chamber, thereby preventing communication ofgases between the intake manifold on that one side of the piston and thefirst combustion chamber.

Simultaneously, the exhaust outlet is in fluid communication with thesecond combustion chamber on the opposite side of the piston, and inletports 44 in the second piston rod portion 43 are located within thesecond combustion chamber. Similarly, when the piston is in anothercombustion stage in the second combustion chamber on the opposite sideof the piston, the piston blocks the exhaust outlet from communicatingwith the second combustion chamber. The closest inlet port 44 to thesecond side of the piston remains outside of the second combustionchamber, thereby preventing communication of gases between the intakemanifold on the second side of the piston and the second combustionchamber. Simultaneously, the exhaust outlet is in fluid communicationwith the first combustion chamber on the first side of the piston, andinlet ports 44 in the first piston rod portion 42 are located within thefirst combustion chamber.

Following an expansion stroke portion, the piston may continue to movein a non-expansion stroke portion for a remainder of the stroke. Thenon-expansion stroke portion of each stroke encompasses the remainingportion of the stroke following the expansion stroke portion. Usefulwork may be extracted from the piston as it moves in the non-expansionstroke portion. The non-expansion stroke portion may include a momentumstroke portion. Useful work may continue to be extracted from the pistonas it moves in the momentum stroke portion. In some embodiments of thedisclosure, substantially the entire momentum stroke portion of thesecond stroke on the second combustion chamber side of the piston maycoincide with compression of gases in the first combustion chamber. Thatis, the momentum that follows an expansion portion of the stroke in onecombustion chamber is used to compress gasses in the other combustionchamber. This may be made possible by an engine structure where an endof an expansion phase in one combustion chamber does not correspond witha combustion point in an opposing combustion chamber. Rather, the enginedesign enables further piston travel (e.g., overshoot) following anexpansion portion of the stroke. In some embodiments, the further pistontravel during the momentum portion of the stroke may be at least a widthof the piston. In other embodiments it may be multiple times a width ofthe piston. In yet other embodiments, it may be at least a half a widthof the piston.

During the momentum stroke portion of each stroke, gases may beexchanged between the combustion chamber where ignition of combustiongases has just occurred and an area external to the cylinder. Theexchange of gases may occur through a passageway in the piston rodportion connected to the piston and extending from a location within theat least one combustion chamber to an area external to the cylinder, andthrough the exhaust ports formed in the peripheral wall of the cylinder.By way of one example with reference to FIGS. 2-7, the positions of thepiston 50 and the piston rod portions 42 are shown during a first strokefrom the far left position of the piston in FIG. 2 to the far rightposition of the piston in FIG. 7. FIGS. 7-12 show the positions of thepiston 50 and the piston rod portions 42 during a second stroke from thefar right position of the piston in FIG. 7 to the far left position ofthe piston in FIG. 12. The far left and far right positions of thepiston in the cylinder 12 may be referred to as combustion points forthe stroke in which the combustion gases have been compressed andignition of the gases at the beginning of a combustion phase isoccurring. When the piston is in the far left position of FIG. 2 andignition is occurring for the combustion gases that have been compressedinto a clearance volume between the left face of the piston and thecylinder head 15 at the left end of the cylinder, the piston is at thecombustion point for the stroke from the left end to the right end ofthe cylinder as viewed in FIGS. 2-7. Similarly, when the piston is inthe far right position of FIG. 7 and ignition is occurring for thecombustion gases that have been compressed into a clearance volumebetween the right face of the piston and the cylinder head 14 at theright end of the cylinder, the piston is at the combustion point for thestroke from the right end to the left end of the cylinder as viewed inFIGS. 7-12.

As the piston continues to move from the combustion point for a strokefrom the left end of the cylinder to the right end of the cylinder, FIG.3 illustrates the piston at a position where the piston has passed thecentrally located exhaust ports 18. At this point, a first combustionchamber on the left side of the piston is now in fluid communicationwith the centrally located exhaust ports 18 and exhaust gases from thecombustion may exit the combustion chamber. Therefore, the expansionstroke portion of the stroke has ended, and the piston is continuing totravel toward the right end of the cylinder in the momentum strokeportion as a result of inertia remaining after the end of the expansionstroke.

As shown in FIGS. 3 and 4, the double-faced piston 50, the first pistonrod portion 43 on the left side of the piston and the centrally locatedexhaust ports 18 may be configured such that the double-faced pistonpasses the centrally located exhaust ports 18 as the piston moves fromthe left end of the cylinder toward the right end of the cylinder beforethe inlet ports 44 closest to the left face of the piston enter thefirst combustion chamber on the left side of the piston. As shown inFIG. 4, the piston 50 has moved completely to the right of the centrallylocated exhaust ports 18 by the time inlet ports 44 in the left pistonrod portion 42 are entering the combustion chamber on the left side ofthe piston to permit gas flow between the combustion chamber and theinlet ports 44. This relative sizing and spacing of the variouscomponents allows exhaust gases generated in the first combustionchamber to begin exiting from the centrally located exhaust ports 18before fresh air or other combustion gases are introduced into the firstcombustion chamber through the piston rod portion 43 on the left side ofthe piston. In some embodiments, air may be pre-compressed and suppliedto respective combustion chambers through piston rod portions 42, 43. Invarious alternative embodiments, the precise placement of the inletports through piston rod portions 42, 43 relative to the opposite facesof the double-faced piston may be varied such that the closest inletport to each face of the piston enters the respective combustion chamberon the same side of the piston shortly after the face of the piston haspassed the near edge of the centrally located exhaust ports, therebyallowing exhaust gases to begin exiting the respective combustionchamber a short time before introduction of the fresh pre-compressed airor other combustion gases (see e.g., FIGS. 4 and 9).

Shortly after the piston has passed the centrally located exhaust ports18 during the momentum stroke portion of the stroke from the left end ofthe cylinder to the right end of the cylinder, as shown in FIG. 4, theedges of the inlet ports 44 in the piston rod portion 43 that areclosest to the left face of the piston start to enter the leftcombustion chamber. At this point a scavenging phase may occur on theleft side of the piston as a result of gases being introduced into theleft combustion chamber through the piston rod portion 43 and inletports 44. The inlet ports 44 are configured such that when the piston isin the momentum stroke portion of the first stroke from the left end tothe right end of the cylinder, gas flow may be continuously communicatedbetween the left combustion chamber and an area external to thecylinder. In the exemplary embodiment shown in the figures, fresh airmay be introduced into the left combustion chamber from the intakemanifold 26 located opposite the cylinder head or integral with thecylinder head on the left end of the cylinder. Simultaneously, exhaustgases may be scavenged from the left combustion chamber by the incomingair or other gases and forced out of the centrally located exhaust ports18.

Some aspects of the disclosure may involve the cylinder and thedouble-faced piston being sized such that the expansion stroke portionof the first stroke on a first side of the piston as the piston movesfrom the first end of the cylinder to the second end of the cylindercoincides with at least one of a scavenging phase and a gas boost phaseon a second side of the piston. A similar coincidence may occur inconnection with the second stroke. By way of non-limiting example withreference to the figures, as the piston continues to move toward theright end of the cylinder, as shown in FIGS. 5 and 6, gas flow may becontinuously communicated between the left combustion chamber and anarea external to the cylinder. The continuous flow of air or other gasesintroduced from the inlet manifold 26 into the combustion chamber mayassist with cooling of the cylinder as well as scavenging of exhaustgases from the combustion chamber, and boosting the gas pressure withinthe left combustion chamber. A similar coincidence is illustrated forthe second stroke in FIGS. 11 and 12. In some embodiments, thecoincidence of compression on one side with scavenging and gas boost onthe other side may precisely correspond. In other embodiments they maysubstantially overlap.

Some aspects of the disclosure may involve the cylinder and thedouble-faced piston being sized such that the momentum stroke portion ofthe first stroke on a first side of the piston as the piston moves fromthe first end of the cylinder to the second end of the cylindercoincides with a compression phase in the second combustion chamber on asecond side of the piston. The first stroke may include a compressionstroke portion in which gases in a combustion chamber are compressed bythe piston. By way of non-limiting example, simultaneously with themomentum stroke portion of the first stroke from the left end of thecylinder to the right end of the cylinder, after the piston has movedpast the centrally located exhaust ports 18 toward the right end of thecylinder, gases on the right side of the piston are compressed during acompression phase on the right side of the piston. When the piston isall the way to the right, as shown in FIG. 7, the combustion gases onthe right side of the piston will have been compressed into theremaining clearance volume of the right combustion chamber and ignitionwill occur to begin the second stroke.

The first stroke, which may include a work stroke, may include anexpansion stroke portion, a momentum stroke portion, and a compressionstroke portion. The work stroke may correspond to when pressure in thecylinder due to combustion gases may be converted into mechanical work.The work stroke may include portions when the piston is moving directlydue to combustion or when the piston is moving due to its inertiaimparted by combustion (e.g., during a momentum stroke portion). Thework stroke may correspond to when an oscillating mass that may includethe piston has kinetic energy. The momentum stroke portion andcompression stroke portion may overlap with one another. For example,the momentum stroke portion may include the portion of the stroke thatbegins from the moment that expansion gases in one combustion chamberare permitted to escape until when the piston changes direction (e.g.,at the next combustion point). On the other hand, the compression strokeportion may begin when gases begin to be compressed on the opposite sideof the piston.

In the transition between the states of FIG. 7 and FIG. 8, for example,piston 50 moves in a stroke from the right end of cylinder 12 toward theleft end of cylinder 12. At this time, the combustion chamber on theright-hand side of piston 50 may be in an expansion stroke portion. Theexpansion stroke portion may begin with ignition in the right combustionchamber and may end at the moment that the right face of piston 50reaches the right-hand edge of exhaust ports 18 and begins to uncoverexhaust ports 18. Also at this time, the combustion chamber on theleft-hand side of the piston may be in a compression stroke portion. Thecompression stroke portion may begin at the moment that the left face ofpiston 50 reaches the left-hand edge of exhaust ports 18 and coversexhaust ports 18. During the compression stroke portion, the combustionchamber on the left-hand side of piston 50 may be sealed. In this state,gases may be trapped in the left-hand side of piston 50.

During the motion of the piston within the cylinder, there may beprovided an exhaust stroke. The exhaust stroke may include the momentumstroke portion, as discussed above, plus a portion of a next strokewhere the piston travels in an opposite direction. The exhaust strokemay last from when exhaust ports 18 begin to be uncovered to whenexhaust ports 18 are again covered by piston 50. Scavenging may occurduring the exhaust stroke.

In accordance with some embodiments of the disclosure, an exhaust strokemay be very long compared to a compression stroke. The exhaust strokemay be longer than both the expansion stroke and compression stroke.This may allow a greater amount of work to be extracted from a piston.

As best seen by way of non-limiting example in FIGS. 2-12, the cylinder12 and the double-faced piston 50 may be sized such that a totaldistance the piston travels during the first stroke from the left end ofthe cylinder to the right end of the cylinder, or during the secondstroke from the right end of the cylinder to the left end of thecylinder may be substantially greater than a distance the piston 50travels during the expansion stroke portion of either stroke. In someexemplary embodiments, the cylinder and the double-faced piston may besized such that the total distance the piston travels during each strokefrom one end of the cylinder to the opposite end of the cylinder mayexceed the distance the piston travels during the expansion strokeportion of the stroke by at least the length of the piston from one faceto the opposite face. In some exemplary embodiments, the cylinder andthe double-faced piston may be sized such that a total distance thepiston travels in each stroke exceeds by at least the length of thepiston a distance traveled by the piston during compression of gases onone side of the piston. The length of the piston 50 from one face to theopposite face in the exemplary embodiment shown in the figures may beless than ½ of a distance from at least one of the cylinder heads 14 tothe centrally located exhaust ports 18. This configuration and relativesizing of the piston and cylinder allows for a significantly greaterlength of the total stroke for the piston in each direction during whichfresh pre-compressed air or other gases may be introduced into thecylinder for the purposes of scavenging exhaust gases and cooling thecylinder after each combustion occurs at opposite ends of the cylinder.

At the beginning of an expansion stroke portion of a stroke from theright end of the cylinder to the left end, as shown in FIG. 7, gas flowmay occur between the combustion chamber on the left side of the pistonand the inlet manifold 26 on the left side of the cylinder, and betweenthe combustion chamber on the left side of the piston and the exhaustmanifold 20 through the exhaust ports 18. The communication of gasesbetween the combustion chamber on the left side of the piston and theexhaust manifold may continue until the left face of the piston hasmoved past the centrally located exhaust ports 18, acting as an exhaustvalve and shutting off communication between the left combustion chamberand the exhaust manifold. Additionally, before the piston 50 has evenclosed off the exhaust ports 18, the inlet ports 44 closest to the leftface of the piston will have moved outside of the left combustionchamber, thereby closing off communication of gases between the leftinlet manifold 26 and the left combustion chamber through the leftpiston rod portion 43.

The length of the double-faced piston 50, the length of the cylinder 12,the location of the exhaust outlets 18, and the location of the inletports 44 in each of the first and second piston rod portions 42, 43extending from opposite faces of the piston 50 may be arranged such thatwhen the piston is in a combustion stage in the second combustionchamber on the right side of the piston, the piston blocks the exhaustoutlet from communicating with the second combustion chamber. Theclosest inlet port 44 to the right side of the piston remains outside ofthe second combustion chamber, thereby preventing communication of gasesbetween the intake manifold on the right side of the piston and thesecond combustion chamber. Simultaneously, the exhaust outlet is influid communication with the first combustion chamber on the left sideof the piston, and inlet ports 44 in the left piston rod portion 43 arelocated within the first combustion chamber.

The momentum stroke portion of each stroke encompasses the remainingportion of the stroke following the expansion stroke portion. During themomentum stroke portion of each stroke, gases may be exchanged betweenthe combustion chamber where ignition of combustion gases has justoccurred and an area external to the cylinder. The exchange of gases mayoccur through a passageway in the piston rod portion connected to thepiston and extending from a location within the at least one combustionchamber to an area external to the cylinder, and through the exhaustports formed in the peripheral wall of the cylinder. FIGS. 7-12 show thepositions of the piston 50 and the piston rod portions 42 during asecond stroke from the far right position of the piston in FIG. 7 to thefar left position of the piston in FIG. 12. As discussed above, the farleft and far right positions of the piston in the cylinder 12 may bereferred to as combustion points for the stroke in which the combustiongases have been compressed and ignition of the gases at the beginning ofa combustion phase is occurring. When the piston is in the far rightposition of FIG. 7 and ignition is occurring for the combustion gasesthat have been compressed into a clearance volume between the right faceof the piston and the cylinder head 14 at the right end of the cylinder,the piston is at the combustion point for the stroke from the right endto the left end of the cylinder, as viewed in FIGS. 7-12.

As the piston continues to move from the combustion point for a strokefrom the right end of the cylinder to the left end of the cylinder, FIG.8 illustrates the piston at a position where the piston has just passedthe centrally located exhaust ports 18. At this point, the secondcombustion chamber on the right side of the piston is now in fluidcommunication with the centrally located exhaust ports 18 and exhaustgases from the combustion that occurred on the right side of the pistonduring the expansion stroke portion of the second stroke may start toexit the combustion chamber. Therefore, the expansion stroke portion ofthe second stroke has ended, and the piston is continuing to traveltoward the left end of the cylinder in the momentum stroke portion as aresult of inertia remaining after the end of the expansion stroke.

As shown in FIGS. 8 and 9, the double-faced piston 50, the second pistonrod portion 42 on the right side of the piston and the centrally locatedexhaust ports 18 may be configured such that the double-faced pistonpasses the centrally located exhaust ports 18 as the piston moves fromthe right end of the cylinder toward the left end of the cylinder beforethe inlet ports 44 closest to the right face of the piston enter thesecond combustion chamber on the right side of the piston. As shown inFIG. 9, the piston 50 has moved completely to the left of the centrallylocated exhaust ports 18 by the time inlet ports 44 in the right pistonrod portion 42 are entering the second combustion chamber on the rightside of the piston to permit gas flow between the second combustionchamber and the inlet ports 44. This relative sizing and spacing of thevarious components allows exhaust gases generated in the secondcombustion chamber to begin exiting from the centrally located exhaustports 18 before fresh pre-compressed air or other combustion gases areintroduced into the second combustion chamber through the piston rodportion 42 on the right side of the piston. In various alternativeembodiments, the precise placement of the inlet ports through piston rodportions 42, 43 relative to the opposite faces of the double-facedpiston may be varied such that the closest inlet port to each face ofthe piston enters the respective combustion chamber on the same side ofthe piston shortly after the face of the piston has passed the near edgeof the centrally located exhaust ports, thereby allowing exhaust gasesto begin exiting the respective combustion chamber a short time beforeintroduction of the fresh pre-compressed air or other combustion gases.

Shortly after the piston has passed the centrally located exhaust ports18 during the momentum stroke portion of the stroke from the right endof the cylinder to the left end of the cylinder, as shown in FIG. 9, theedges of the inlet ports 44 in the piston rod portion 42 that areclosest to the right face of the piston start to enter the secondcombustion chamber. At this point a scavenging phase may occur on theright side of the piston as a result of pre-compressed gases beingintroduced into the second combustion chamber through the piston rodportion 42 and inlet ports 44. The inlet ports 44 are configured suchthat when the piston is in the momentum stroke portion of the secondstroke from the right end to the left end of the cylinder, gas flow maybe continuously communicated between the second combustion chamber andan area external to the cylinder. In the exemplary embodiment shown inthe figures, fresh, pre-compressed air may be introduced into the secondcombustion chamber from the intake manifold 26 located opposite thecylinder head or integral with the cylinder head on the right end of thecylinder. Simultaneously, exhaust gases may be scavenged from the secondcombustion chamber on the right side of the piston 50 by the incomingpre-compressed air or other gases and forced out of the centrallylocated exhaust ports 18.

As the piston continues to move toward the left end of the cylinder, asshown in FIGS. 10 and 11, gas flow may be continuously communicatedbetween the second combustion chamber and an area external to thecylinder. The continuous flow of pre-compressed air or other gasesintroduced from the inlet manifold 26 into the second combustion chambermay assist with cooling of the cylinder as well as scavenging of exhaustgases from the second combustion chamber, and boosting the gas pressurewithin the second combustion chamber. Simultaneously with the momentumstroke portion of the second stroke from the right end of the cylinderto the left end of the cylinder, after the piston has moved past thecentrally located exhaust ports 18 toward the left end of the cylinder,gases on the left side of the piston are compressed during a compressionphase on the left side of the piston. When the piston is all the way tothe left, as shown in FIG. 2, the combustion gases on the left side ofthe piston will have been compressed into the remaining clearance volumeof the left combustion chamber and ignition will occur to begin anotherstroke from the left end of the cylinder to the right end of thecylinder.

In accordance with some embodiments of the disclosure, regardless ofother particular structures in the engine, a cylinder and a double-facedpiston may be sized such that a total distance the piston travels duringa first stroke is substantially greater than a distance the pistontravels during an expansion stroke portion of the first stroke. By wayof example with reference to FIGS. 7-12, the total distance of pistontravel may be measured from the combustion point on the right side ofthe engine 10, as illustrated in FIG. 7, to the combustion point on theleft side of engine 10, as illustrated in FIG. 12. This total distancetraveled is substantially greater than the expansion portion of thestroke which occurs when, in the progression of FIGS. 7-12, the piston50 passes at least one of the exhaust ports 18. It is contemplated thatin other embodiments of the disclosure, the end of the expansion strokemight be marked by other occurrences, such as the opening of amechanical valve, or the cessation of expansion in some other manner.Regardless of how the expansion stroke portion ends, such embodimentsare contemplated to be within the scope of this disclosure so long asthe total distance of travel is substantially greater than the expansionportion alone. By way of non-limiting examples, the total distance maybe considered substantially greater if the difference between theexpansion portion of the stroke and a non-expansion portion of thestroke is either multiple times the width of the piston, the width ofthe piston, greater than three quarters the width of the piston, greaterthan half the width of the piston, or greater than a quarter width ofthe piston. Thus, for example, the double-faced piston may have an axiallength from one face of the piston to an opposite face of the pistonthat is less than or equal to ½ of a distance from at least one of thefirst cylinder head and the second cylinder head to the exhaust port.

In some exemplary embodiments the cylinder and the double-faced pistonmay be sized such that the total distance the piston travels during eachstroke from one end of the cylinder to the opposite end of the cylindermay exceed the distance the piston travels during the expansion strokeportion of the stroke by at least the length of the piston from one faceto the opposite face. In other exemplary embodiments the cylinder andthe double-faced piston may be sized such that a total distance thepiston travels in each stroke exceeds by at least the length of thepiston a distance traveled by the piston during compression of gases onone side of the piston. The length of the piston 50 from one face to theopposite face in the exemplary embodiment shown in the figures may beless than ½ of a distance from at least one of the cylinder heads 14 tothe centrally located exhaust ports 18. This configuration and relativesizing of the piston and cylinder may allow for a significantly greaterlength of the total stroke for the piston in each direction during whichfresh pre-compressed air or other gases may be introduced into thecylinder for the purposes of scavenging exhaust gases and cooling thecylinder after each combustion occurs at opposite ends of the cylinder.

In accordance with some embodiments of the disclosure, an internalcombustion engine may include a piston being formed of an assembly ofseparate pieces, including a pair of piston end disks, each having afirst outer diameter, and wherein the center disk is configured to causea thermal gap between the pair of piston end disks. By way of example,and as shown in FIGS. 13 to 22, various embodiments of an engineaccording to this disclosure may include a double-faced piston 50. Thepiston 50 may include a cylindrical first piston portion 56 having afirst diameter, a cylindrical second piston portion 54 of the firstdiameter, and a cylindrical third piston portion 52 of a second diameterless than the first diameter. The cylindrical third piston portion 52may be located between the first piston portion 56 and the second pistonportion 54, and the first piston portion 56 may be configured such thatprior to assembly, the first piston portion 56 is separate from thesecond piston portion 52.

In accordance with some embodiments, the hardness of the center diskdiffers from the hardness of the end disks. In addition, oralternatively, the piston center disk may be integrally formed with oneof the pair of piston end disks.

Embodiments may also include a continuous, gapless piston ringcircumscribing a piston portion, where the piston ring is configuredsuch that when heated the piston ring deforms in an axial direction ofthe piston. Variously shaped piston rings may be employed consistentwith embodiments of the disclosure. Such shapes may include a wavepattern or other meandering constructions that are either symmetrical ornon-symmetrical. As illustrated by way of example only in FIG. 20, acontinuous, gapless piston ring 64 may circumscribe the third pistonportion 52, where the piston ring 64 is configured such that whenheated, the piston ring deforms in an axial direction of the piston 50.The third piston portion 52 may define a slot between the first pistonportion 56 and the second piston portion 54. The slot defined betweenthe first piston portion 56 and the second piston portion 54 may alsoform a thermal gap that is not completely filled by the piston ring, andthat therefore facilitates heat transfer away from the piston ring,thereby increasing its longevity. In some embodiments prior to assembly,the third piston portion 52 may be integral with the first pistonportion 56, and the second piston portion 54 may be non-integral withthe third piston portion 52.

As shown in FIG. 13, a groove in the outer peripheral wall of the piston50 may be defined by the assembly of the first, second, and third pistonportions, as described above, or may be machined or otherwisemanufactured, e.g., using 3D additive manufacturing processes. Thegroove may include a first edge and a second edge spaced from the firstedge. A piston ring 64 (FIGS. 17-20) may be installed in the groove, andthe piston ring may have a shape that meanders within the groove, suchthat the shape of the piston ring differs from a shape of the groove andsuch that the piston ring does not substantially fill the groove. Thepiston ring 64 may be constructed of a material that when subjected toheat causes a shape of the meanderings to change, thereby enabling thepiston ring to expand in an axial direction of the piston, between theedges of the groove. As best seen in FIGS. 17, 19, and 20, themeanderings of the piston ring 64 may be in the shape of a wave. Peaksof the wave alternatively extend toward opposing edges of the groove.The piston ring 64 may be constructed such that when subjected to heat,the piston ring tends to expand in an axial direction of the pistonrather than radially.

As shown in FIGS. 17-20, the piston ring 64 may have an undulating axialcross section and a circular radial cross section. The piston ring 64may include a plurality of staggered, flat abutment surface portions 68on axially opposite faces. The flat abutment surface portions 68 may beconfigured to seat alternately on opposite edges of the groove. A gapbetween the first and second edges of the groove of the piston 50 mayallow for axially-directed expansion and contraction of the piston ring64 while maintaining a circular radial cross section of the piston ringhaving a substantially constant outer diameter 70 that remains in fullcontact with an inner peripheral wall of the cylinder 12 at all times.

In a plan view of the piston ring 64, as can be clearly seen in FIG. 19,the piston ring 64 is round, in order to fit tightly against a cylinderwall 66. In one exemplary embodiment, each side of the piston ring 64may be provided with six evenly peripherally distributed flat abutmentsurface portions 68 for abutting the piston ring 64 against the adjacentpiston portion, i.e., the first piston portion 56 and the second pistonportion 54. The abutment surface portions 68 of one side of the pistonring 64 may be angularly shifted with respect to the abutment surfaceportions 68 of the other side of the piston ring 64, such that eachabutment surface portion 68 of one side of the piston ring 64 is equallydistanced from the two adjacent abutment surface portions 68 of theother side of the piston ring 64.

As can be seen in FIG. 18, which is a side view of the piston ring 64, acurved ring wall 69 may be formed between two adjacent abutment surfaceportions 68 of both sides of the piston ring 64.

Depending on construction and materials employed, in some embodimentsthe above described structure of the piston ring 64 may have severaladvantages. The piston ring 64 is peripherally continuous, in contrastto traditional piston rings, thus substantially eliminating compressionlosses during the operation of the engine due to leakage of compressedgas from one side of the piston ring to an opposite side thereof. As aresult of the reduction in compression losses, a single piston ring 64may be used, rather than two or three piston rings, as known in the art.(although multiple rings consistent with this disclosure may be employedon a single piston consistent with this disclosure.) The reduction inthe number of piston rings may result in a significant reduction infriction losses caused by the sliding contact between each piston ringand the cylinder wall 66. The reduction in friction losses in turn mayresult in improvements in the efficiency of the engine 10. The abutmentsurface portions 68 on both sides of the piston ring 64 may also ensurethat the piston ring 64 will remain directed in an orientationsubstantially perpendicular to the longitudinal axis A, which in turnmay result in the ring peripheral surface 70 remaining parallel to thecylinder wall 66 and in a continuous contact therewith. As the pistonring 64 according to various exemplary embodiments of this disclosure isheated during operation and tends to expand, the ring peripheral surface70 will remain in full contact with the cylinder wall 66, and may exertsubstantially consistent pressure thereon. Expansion and contraction ofthe piston ring 64 may result in an increased curvature andaxially-directed expansion of the curved ring walls 69, therebyabsorbing the expansion without disturbing the constant radial profileof the piston ring 64.

The engine 10 according to the various exemplary embodiments of thisdisclosure may facilitate a nearly continuous scavenging of hot exhaustgases from the engine while continuously supplying fresh air forcombustion. The nearly continuously introduced fresh pre-compressed airmay decrease the temperature within the cylinder and increase the engineefficiency and engine service life.

To expedite the foregoing portion of the disclosure, variouscombinations of elements are described together. It is to be understood,that aspects of the invention in their broadest sense are not limited tothe particular combinations previously disclosed. Rather, embodiments ofthe invention, consistent with this disclosure, and as illustrated byway of example only in the Figures, may include one or more of thefollowing, either alone or in combination with any one or more other ofthe following, or in combination with the previously disclosed features:

-   -   an internal combustion engine.    -   a linear reciprocating engine.    -   a cylinder defining at least one combustion chamber in the        engine block.    -   a piston in the cylinder, the piston being configured to travel        in a first stroke from one end of the cylinder to an opposite        end of the cylinder, and being sized relative to the cylinder to        enable an expansion stroke portion of the first stroke wherein        the piston travels under gas expansion pressure, and a momentum        stroke portion of the first stroke for the remainder of the        first stroke following the expansion stroke portion.    -   at least one piston rod portion connected to the piston and        extending from a location within the at least one combustion        chamber to an area external to the cylinder.    -   at least one recess in the piston rod portion, the at least one        recess forming a passageway configured to communicate gas flow        between the at least one combustion chamber and the area        external to the cylinder.    -   wherein the at least one recess is configured such that when the        piston is in the momentum stroke portion of the first stroke        following the expansion stroke portion of the first stroke, the        at least one recess is configured to continuously communicate        gas flow between the at least one combustion chamber and the        area external to the cylinder.    -   wherein the at least one recess forming the passageway renders        the at least one piston rod portion at least partially hollow.    -   wherein the passageway includes a groove in the at least one        piston rod portion.    -   wherein the passageway is configured to introduce combustion gas        into the at least one combustion chamber from a location outside        the cylinder.    -   wherein the piston is double-faced and wherein the at least one        piston rod portion includes a pair of piston rod portions, each        piston rod portion extending from an opposing face of the        double-faced piston.    -   wherein the at least one recess includes a channel extending        internal to the at least one piston rod portion.    -   wherein the pair of piston rod portions are integrally formed.    -   wherein the pair of piston rod portions are indirectly connected        to each other through the double-faced piston.    -   wherein the at least one recess includes at least two recesses,        each extending through a different piston rod portion.    -   further including at least one port in the at least one piston        rod portion and in fluid communication with the passageway.    -   wherein the at least one port includes multiple elongated slots.    -   wherein the at least one port includes multiple holes in the        piston rod.    -   wherein the passageway includes a plurality of grooves formed in        an outer peripheral surface of the at least one piston rod        portion.    -   wherein the at least one recess in the piston rod portion        includes a rod section of reduced diameter.    -   wherein the at least one combustion chamber includes a first        combustion chamber defined between a first end of the piston and        a first end of the cylinder, and a second combustion chamber        defined between a second end of the piston and a second end of        the cylinder.    -   wherein the cylinder is closed at each opposite end by a        cylinder head.    -   wherein the at least one piston rod portion includes a first        piston rod portion extending from the first end of the piston        through the cylinder head at the first end of the cylinder, and        a second piston rod portion extending from the second end of the        piston through the cylinder head at the second end of the        cylinder.    -   wherein the cylinder head at each end of the cylinder includes        an intake manifold, wherein the passageway in the first piston        rod portion is configured to communicate gas flow between the        first combustion chamber and the intake manifold at the first        end of the cylinder, and the passageway in the second piston rod        portion is configured to communicate gas flow between the second        combustion chamber and the intake manifold at the second end of        the cylinder.    -   wherein a peripheral wall of the cylinder between the first and        second ends of the cylinder includes at least one exhaust port.    -   wherein the at least one exhaust port includes a plurality of        exhaust ports spaced around the circumference of the cylinder,        and wherein the plurality of exhaust ports are in fluid        communication with an exhaust manifold.    -   wherein substantially the entire expansion stroke portion of the        first stroke on the first combustion chamber side of the piston        coincides with gas flow between the second combustion chamber        and the intake manifold at the second end of the cylinder.    -   wherein substantially the entire momentum stroke portion of the        first stroke on the first combustion chamber side of the piston        coincides with compression of gases in the second combustion        chamber.    -   wherein the piston is further configured to travel in a second        stroke from the second end of the cylinder to the first end of        the cylinder, and being sized relative to the cylinder to enable        an expansion stroke portion of the second stroke wherein the        piston travels under gas expansion pressure, and a momentum        stroke portion of the second stroke for the remainder of the        second stroke following the expansion stroke portion.    -   wherein substantially the entire expansion stroke portion of the        second stroke on the second combustion chamber side of the        piston coincides with gas flow between the first combustion        chamber and the intake manifold at the first end of the        cylinder.    -   wherein substantially the entire momentum stroke portion of the        second stroke on the second combustion chamber side of the        piston coincides with compression of gases in the first        combustion chamber.    -   a double-faced piston slidably mounted within the cylinder and        configured to move in a first stroke from the first end of the        cylinder to the second end of the cylinder, wherein the        double-faced piston and the cylinder are configured such that        the first stroke includes an expansion stroke portion during        which chemical energy from combustion in the first combustion        chamber is converted into mechanical power of the piston, and a        momentum stroke portion during which the piston continues to        move to the second end of the cylinder and gases are exchanged        between the first combustion chamber and a location outside the        cylinder.    -   wherein the cylinder and the double-faced piston are sized such        that a total distance the piston travels during the first stroke        is substantially greater than a distance the piston travels        during the expansion stroke portion of the first stroke.    -   wherein the cylinder and the double-faced piston are sized such        that the total distance the piston travels during the first        stroke exceeds the distance the piston travels during the        expansion stroke portion of the first stroke by at least the        length of the piston from one face to the opposite face.    -   wherein the cylinder and the double-faced piston are sized such        that the expansion stroke portion of the first stroke on a first        side of the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with at        least one of a scavenging phase and a gas boost phase on a        second side of the piston.    -   wherein the cylinder and the double-faced piston are sized such        that the momentum stroke portion of the first stroke on a first        side of the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with a        compression phase in the second combustion chamber on a second        side of the piston.    -   wherein the double-faced piston is configured to move in a        second stroke from the second end of the cylinder to the first        end of the cylinder, and wherein the cylinder and the        double-faced piston are sized such that the second stroke        includes an expansion stroke portion during which chemical        energy from combustion in the second combustion chamber is        converted into mechanical power of the piston, and a momentum        stroke portion during which the piston continues to move to the        first end of the cylinder and gases are exchanged between the        second combustion chamber and a location outside the cylinder.    -   wherein the cylinder and the piston are sized such that a total        distance the piston travels during the second stroke is        substantially greater than a distance the piston travels during        the expansion portion of the second stroke.    -   wherein the total distance the piston travels during the second        stroke exceeds the distance the piston travels during the        expansion stroke portion of the second stroke by at least the        length of the piston from one face to the opposite face.    -   wherein the expansion stroke portion of the second stroke on a        second side of the piston as the piston moves from the second        end of the cylinder to the first end of the cylinder coincides        with at least one of a scavenging phase and a gas boost phase on        a first side of the piston.    -   wherein the momentum portion of the second stroke on a second        side of the piston as the piston moves from the second end of        the cylinder to the first end of the cylinder coincides with a        compression phase in the first combustion chamber on a first        side of the piston.    -   a first piston rod portion connected to a first face of the        double-faced piston and extending from a location within the        first combustion chamber to a first location outside the        cylinder.    -   a second piston rod portion connected to a second face of the        double-faced piston and extending from a location within the        second combustion chamber to a second location outside the        cylinder.    -   at least one recess in the first piston rod portion, the at        least one recess forming a passageway configured to communicate        gas flow between the first combustion chamber and the first        location outside the cylinder.    -   at least one recess in the second piston rod portion, the at        least one recess forming a passageway configured to communicate        gas flow between the second combustion chamber and the second        location outside the cylinder.    -   at least one port in a peripheral side wall of the cylinder, the        at least one port being configured to communicate gas flow        between the first combustion chamber and outside the cylinder        when the piston is on the second combustion chamber side of the        at least one port, and being configured to communicate gas flow        between the second combustion chamber and outside the cylinder        when the piston is on the first combustion chamber side of the        at least one port.    -   wherein the passageways in the first and second piston rod        portions are configured to intake gases into the first and        second combustion chambers, respectively, and the at least one        port in a peripheral side wall of the cylinder is configured to        exhaust gases from the first and second combustion chambers,        respectively.    -   wherein each of the first stroke and the second stroke includes        an expansion stroke portion during which chemical energy from        combustion in one of the first combustion chamber and the second        combustion chamber is converted into mechanical power of the        piston, and a momentum stroke portion during which the piston        continues to move toward a respective end of the cylinder and        gases are exchanged between one of the first combustion chamber        and the second combustion chamber and a location outside the        cylinder.    -   wherein the cylinder and the piston are sized such that a total        distance the piston travels in each of the first and second        strokes exceeds by at least a length of the piston a distance        traveled by the piston during compression of gases on one side        of the piston.    -   wherein the cylinder and the piston are sized such that an        expansion stroke portion of the first stroke on a first side of        the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with at        least one of a scavenging phase and a gas boost phase on a        second side of the piston.    -   wherein the cylinder and the piston are sized such that a        momentum stroke portion of the first stroke on a first side of        the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with        compression of gases in the second combustion chamber on a        second side of the piston.    -   at least one port in a peripheral side wall of the cylinder, the        at least one port being configured to communicate gas flow        between the first combustion chamber and outside the cylinder        when the piston is on the second combustion chamber side of the        at least one port, and being configured to communicate gas flow        between the second combustion chamber and outside the cylinder        when the piston is on the first combustion chamber side of the        at least one port.    -   wherein the passageways in the first and second piston rod        portions are configured to intake gases into the first and        second combustion chambers, respectively, and the at least one        port in a peripheral side wall of the cylinder is configured to        exhaust gases from the first and second combustion chambers,        respectively.    -   wherein the first passageway and the second passageway are        configured to prevent gases from being exchanged between the        cylinder and a location outside the cylinder via a path that        crosses the first face and the second face.    -   wherein the first passageway and the second passageway render        the first and second piston rod portions at least partially        hollow.    -   wherein at least one of the first and second passageways        includes a groove in the respective first and second piston rod        portions.    -   wherein the first and second passageways are configured to        introduce combustion gas into the first and second combustion        chambers, respectively, from a location outside the cylinder.    -   wherein the first and second passageways include elongated        channels extending internal to the first and second piston rod        portions.    -   wherein the first and second piston rod portions are integrally        formed.    -   wherein the first and second piston rod portions are indirectly        connected to each other through the double-faced piston.    -   further including at least one port in the first piston rod        portion in fluid communication with the first passageway and at        least one port in the second piston rod portion in fluid        communication with the second passageway.    -   wherein the at least one port in the first and second piston rod        portions includes multiple elongated slots.    -   wherein the at least one port in the first and second piston rod        portions includes multiple holes in the piston rod portions.    -   wherein at least one of the first and second passageways in the        first and second piston rod portions includes a plurality of        grooves formed in an outer peripheral surface of the respective        piston rod portion.    -   wherein at least one of the first and second recesses in the        first and second piston rod portions includes a piston rod        section of reduced diameter.    -   wherein the first passageway in the first piston rod portion and        the second passageway in the second piston rod portion are        configured to intake gases into the first and second combustion        chambers, respectively.    -   a first piston rod portion extending from a first face of the        double-faced piston through the first combustion chamber and        through the first cylinder head.    -   a first recess in the first piston rod portion defining a first        passageway for communicating gas between the first combustion        chamber and a first location external to the cylinder.    -   a second piston rod portion extending from a second face of the        piston through the second combustion chamber and through the        second cylinder head.    -   a second recess in the second piston rod portion defining a        second passageway for communicating gas between the second        combustion chamber and a second location external to the        cylinder.    -   at least one port in a peripheral wall of the cylinder, for        alternatively communicating gases between at least one region        external to the cylinder and at least one of the first        combustion chamber and the second combustion chamber.    -   wherein the double-faced piston, the first piston rod portion,        and the at least one port are configured such that the        double-faced piston passes the at least one port as the piston        moves from the first position toward the second position before        an opening of the first recess enters the first combustion        chamber to thereby permit gas flow between the first combustion        chamber and the first recess in the first piston rod portion.    -   wherein the double-faced piston, the second piston rod portion,        and the at least one port are configured such that the        double-faced piston passes the at least one port as the piston        moves from the second position toward the first position before        an opening of the second recess enters the second combustion        chamber to thereby permit gas flow between the second combustion        chamber and the second recess in the second piston rod portion.    -   wherein the first recess in the first piston rod portion and the        second recess in the second piston rod portion are configured as        inlets for the intake of gases, and the at least one port in the        peripheral wall is configured as an outlet for the exhaust of        gases.    -   further including at least one additional recess in the first        piston rod portion and at least one additional recess in the        second piston rod portion.    -   wherein the double-faced piston, the first piston rod portion,        and the at least one port in the peripheral cylinder wall are        configured such that when the double-faced piston is located        between the first cylinder head and the at least one port in the        peripheral wall, an opening of the first recess is outside the        cylinder and the double-faced piston blocks gas flow between the        first combustion chamber and the at least one port, and wherein        the double-faced piston, the second piston rod portion, and the        at least one port in the peripheral cylinder wall are configured        such that when the double-faced piston is located between the        second cylinder head and the at least one port in the peripheral        wall, an opening of the second recess is outside the cylinder        and the double-faced piston blocks gas flow between the second        combustion chamber and the at least one port in the peripheral        wall.    -   wherein the recesses in the first piston rod portion and the        second piston rod portion include a bore through a respective        core of each of the first piston rod portion and the second        piston rod portion.    -   wherein the openings of the recesses in the first piston rod        portion and the second piston rod portions include a curvilinear        port in a respective outer wall of each respective piston rod        portion.    -   wherein the openings of the recesses in the first piston rod        portion and the second piston rod portions include an elongated        slot in a respective outer wall of each respective piston rod        portion.    -   wherein the recesses in the first and second piston rod portions        are defined by regions of reduced diameter.    -   wherein the at least one port includes an exhaust port located        in an axially central region of the cylinder peripheral wall.    -   wherein during compression and combustion of gases in one of the        first and second combustion chambers, the piston acts as an        exhaust valve preventing the flow of exhaust gases out of the        one of the combustion chambers while enabling the flow of        exhaust gases out of the other of the combustion chambers.    -   an exhaust port located in a peripheral wall of the cylinder at        a generally central region of the cylinder between the first        cylinder head and the second cylinder head.    -   at least one combustion gas inlet in a location other than the        peripheral cylinder wall, wherein the combustion gas inlet and        the exhaust port are configured to cooperate such that        combustion gases introduced through the inlet are evacuated from        the cylinder through the exhaust port in the peripheral wall.    -   wherein the double-faced piston has an axial length from one        face of the piston to an opposite face of the piston that is        less than or equal to ½ of a distance from at least one of the        first cylinder head and the second cylinder head to the exhaust        port.    -   further including a first piston rod portion extending from a        first face of the double-faced piston through the first        combustion chamber and through the first cylinder head, and        wherein the at least one combustion gas inlet is located in the        first piston rod portion.    -   further including a second piston rod portion extending from a        second face of the double-faced piston through the second        combustion chamber and through the second cylinder head, and        wherein the at least one combustion gas inlet is located in the        second piston rod portion.    -   wherein the at least one combustion gas inlet includes a first        passageway in fluid communication with a first intake manifold        located adjacent the first cylinder head and a second passageway        in fluid communication with a second intake manifold located        adjacent the second cylinder head.    -   a first elongated channel in the first piston rod portion        configured to serve as an intake inlet for gas from a location        external to the cylinder, through the first end of the first        combustion chamber to a location within the first combustion        chamber.    -   a second elongated channel in the second piston rod portion        configured to serve as an intake inlet for gas from a location        external to the cylinder, through the second end of the second        combustion chamber to a location within the second combustion        chamber.    -   wherein a length of the double-faced piston, a length of the        cylinder, a location of the exhaust outlet, and a location of a        channel access opening in each of the first and second piston        rod portions are arranged such that when the piston is in a        combustion stage in the first combustion chamber, the piston        blocks the exhaust outlet from communicating with the first        combustion chamber and the channel access opening in the first        piston rod portion is outside of the first combustion chamber,        while simultaneously the exhaust outlet is in fluid        communication with the second combustion chamber, and the access        opening of the second channel is within the second combustion        chamber.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet are configured such that        scavenging of combustion gases from the first combustion chamber        occurs through the exhaust outlet when a channel access opening        in the first piston rod portion is located within the first        combustion chamber and the piston is in a position on the second        combustion chamber side of the exhaust outlet.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet are configured such that gas        boost in the first combustion chamber follows scavenging of        combustion gases from the first combustion chamber as        pre-charged air continues to be introduced through the channel        access opening in the first piston rod portion into the first        combustion chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet are configured such that        compression of gases within the second combustion chamber occurs        when the piston is in a position past the exhaust outlet toward        the second end of the second combustion chamber and the channel        access opening in the second piston rod portion is outside of        the second combustion chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet are configured such that        scavenging of combustion gases from the second combustion        chamber occurs through the exhaust outlet when the channel        access opening in the second piston rod portion is in the second        combustion chamber and the piston is in a position past the        exhaust outlet toward the first end of the first combustion        chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet is configured such that gas boost        in the second combustion chamber follows scavenging of        combustion gases from the second combustion chamber as        pre-charged air continues to be introduced through the channel        access opening in the second piston rod portion into the second        combustion chamber.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet is configured such that        compression of gases within the first combustion chamber occurs        when the piston is in a position past the exhaust toward the        first end of the first combustion chamber and the access opening        in the first piston rod portion is outside of the first        combustion chamber.    -   wherein a compression ratio of the engine is a function of at        least one of a closest spacing between a channel access opening        in the first piston rod portion and the first face of the        double-faced piston, and the closest spacing between a channel        access opening in the second piston rod portion and the second        face of the double-face piston.    -   A piston for an internal combustion engine, the piston including        a cylindrical first piston portion having a first diameter, a        cylindrical second piston portion of the first diameter, a        cylindrical third piston portion of a second diameter less than        the first diameter, and located between the first piston portion        and the second piston portion, and wherein the first piston        portion is configured such that prior to assembly, the first        piston portion is separate from the second piston portion, a        continuous, gapless piston ring circumscribing the third piston        portion, where the piston ring is configured such that when        heated the piston ring deforms in an axial direction of the        piston.    -   wherein the third piston portion defines a slot between the        first piston portion and the second piston portion.    -   wherein, prior to assembly, the third piston portion is integral        with the first piston portion, and the second piston portion is        non-integral with the third piston portion.    -   a groove in the outer peripheral wall of the piston the groove        having a first edge and a second edge spaced from the first        edge.    -   a piston ring in the groove, the piston ring having a shape that        meanders within the groove, such that the shape of the piston        ring differs from a shape of the groove and such that the piston        ring does not substantially fill the groove, and wherein the        piston ring is constructed of a material that when subjected to        heat causes a shape of the meanderings to change, thereby        enabling the piston ring to expand in an axial direction of the        piston, between the edges of the groove.    -   wherein the meanderings are in the shape of a wave.    -   wherein peaks of the wave alternatively extend toward opposing        edges of the groove.    -   wherein the piston ring is constructed such that when subjected        to heat, the piston ring tends to expand in an axial direction        of the piston rather than radially.    -   wherein the piston ring has an undulating axial cross section        and a circular radial cross section.    -   wherein the piston ring includes a plurality of staggered, flat        abutment surface portions on axially opposite faces.    -   wherein the flat abutment surface portions are configured to        seat alternately on opposite edges of the groove.    -   wherein a gap between the first and second edges of the groove        allows for axially-directed expansion and contraction of the        piston ring while maintaining a circular radial cross section of        the piston ring having a substantially constant outer diameter.    -   wherein the piston ring is formed with an undulating axial cross        section including a plurality of staggered, flat abutment        surface portions on axially opposite faces thereof, the flat        abutment surface portions being configured to seat alternately        on the first and second edges of the groove with portions of the        piston ring in between the flat abutment surface portions being        spaced from the edges of the groove.    -   a piston formed of an assembly of separate pieces including a        pair of piston end disks each having a first outer diameter, a        piston center disk having a second outer diameter smaller than        the first outer diameter, and wherein the center disk is        configured to cause a thermal gap between the pair of piston end        disks.    -   further including the piston center disk having a hardness that        is different from the piston end disks.    -   wherein the piston center disk is integrally formed with one of        the pair of piston end disks.

Various alterations and modifications may be made to the disclosedexemplary embodiments without departing from the spirit or scope of thedisclosure as embodied in the following claims. For example, the burnedgases produced by the engine 10 may be used for driving a turbo charger.The compressed air introduced into the cylinder may be pressurized by anexternal compressor that is driven by the reciprocating piston rodportions extending from opposite ends of the cylinder. Other variationsmay include imparting a swirl effect to the gases introduced into thecylinder by changing the angle of the inlet ports and of the outletports so that gases are not directed radially into or out of thecylinder.

What is claimed is:
 1. An internal combustion engine, comprising: acylinder having a first combustion chamber at a first end and a secondcombustion chamber at a second end; an exhaust port provided in a wallof the cylinder; a first cylinder head located at an end of the firstcombustion chamber; a second cylinder head located at an end of thesecond combustion chamber; a double-faced piston slidably mounted withinthe cylinder and configured to move in a first stroke from the first endof the cylinder to the second end of the cylinder, wherein the firststroke includes a first expansion stroke portion during which chemicalenergy from combustion in the first combustion chamber is converted intomechanical power of the piston, and a first non-expansion stroke portionduring which the piston continues to move toward the second end of thecylinder; a first piston rod portion extending from a first face of thepiston within the first combustion chamber to a first location outsidethe cylinder; a second piston rod portion extending from an opposingsecond face of the piston within the second combustion chamber to asecond location outside the cylinder; a recess in the first piston rodportion or the second piston rod portion forming a path configured tocommunicate gases between the first combustion chamber and the firstlocation outside the cylinder or between the second combustion chamberand the second location outside the cylinder; a chamber surrounding thefirst piston rod portion, the chamber configured to be supplied with gasand the chamber being isolated from the first combustion chamber and thesecond combustion chamber.
 2. The engine of claim 1, wherein the chamberis configured to recirculate gases into the first combustion chamber orthe second combustion chamber.
 3. The engine of claim 1, wherein thefirst non-expansion stroke portion includes a scavenging phase duringwhich gases in the chamber are supplied to the first combustion chamberor the second combustion chamber.
 4. The engine of claim 1, wherein thecylinder and the piston are sized such that, during the firstnon-expansion stroke portion, the piston moves past the exhaust port andthe recess is brought into communication with the first combustionchamber or the second combustion chamber, for at least a length of thefirst stroke equal to a length of the piston.
 5. The engine of claim 1,wherein the recess includes: a first recess in the first piston rodportion forming a first passageway configured to communicate gasesbetween the first combustion chamber and the first location outside thecylinder; and a second recess in the second piston rod portion forming asecond passageway configured to communicate gases between the secondcombustion chamber and the second location outside the cylinder.
 6. Theengine of claim 1, wherein the cylinder and the piston are sized suchthat a total distance of the first stroke is substantially greater thanthat of the first expansion stroke portion or the first non-expansionstroke portion.
 7. The engine of claim 1, wherein the chamber isconfigured to be supplied with air that is to be drawn into the firstcombustion chamber or the second combustion chamber.
 8. The engine ofclaim 1, wherein the engine is a linear reciprocating engine.
 9. Theengine of claim 1, further comprising a second chamber surrounding theother of the first piston rod portion or the second piston rod portion.10. The engine of claim 1, wherein the chamber is configured to have asubstantially constant volume while the piston moves in the firststroke.
 11. The engine of claim 1, wherein the first piston rod portionextends beyond the chamber.
 12. A linear reciprocating engine,comprising: a cylinder having a first combustion chamber at a first endthereof and a second combustion chamber at an opposing second endthereof; a first cylinder head located at an end of the first combustionchamber; a second cylinder head located at an end of the secondcombustion chamber; a double-faced piston slidably mounted within thecylinder and configured to travel in a first stroke from a firstposition in a region of the first combustion chamber to a secondposition in a region of the second combustion chamber, the pistonincluding a first face and a second face; a first piston rod portionextending from the first face of the piston through the first combustionchamber, the first piston rod portion having at least one port; a secondpiston rod portion extending from a second face of the piston throughthe second combustion chamber, the second piston rod portion having atleast one port; a first recess in the first piston rod portionconfigured to communicate gases between the first combustion chamber anda first location outside the cylinder opposite the first cylinder head,the first recess being in fluid communication with the at least one portof the first piston rod portion; a second recess in the second pistonrod portion configured to communicate gases between the secondcombustion chamber and a second location outside the cylinder oppositethe second cylinder head, the second recess being in fluid communicationwith the at least one port of the second piston rod portion; aperipheral port in a wall of the cylinder, wherein the peripheral portis configured such that when the piston is in the first combustionchamber, the peripheral port communicates with the second combustionchamber, and when the piston is in the second combustion chamber, theperipheral port communicates with the first combustion chamber; and achamber surrounding the first piston rod portion or the second pistonrod portion, the chamber configured to be supplied with gas and thechamber being isolated from the first combustion chamber and the secondcombustion chamber.
 13. The engine of claim 12, wherein the chamber isconfigured to recirculate gases into the first combustion chamber or thesecond combustion chamber.
 14. The engine of claim 12, wherein thepiston is configured to move in a non-expansion stroke portion followingthe expansion stage, the non-expansion stroke portion including ascavenging phase during which gases in the chamber are supplied to thefirst combustion chamber or the second combustion chamber.
 15. Theengine of claim 12, wherein the piston, the peripheral port and thecylinder are configured such that following an expansion stage in thefirst combustion chamber or the second combustion chamber, the pistonmoves beyond one side of the peripheral port by a distance greater thanor equal to the width of the piston, such that gases from the chamberare supplied to the first combustion chamber or the second combustionchamber.
 16. The engine of claim 12, wherein the peripheral portincludes a plurality of openings in the wall of the cylinder, the atleast one port of the first piston rod portion includes a plurality ofopenings in the first piston rod portion, and the at least one port ofthe second piston rod portion includes a plurality of openings in thesecond piston rod portion.
 17. The engine of claim 12, wherein thecylinder and the piston are sized such that a total distance of a strokefrom the first end of the cylinder to the second end of the cylinder issubstantially greater than that of the expansion stage.
 18. The engineof claim 12, wherein the chamber is configured to be supplied with airthat is to be drawn into the first combustion chamber or the secondcombustion chamber.
 19. The engine of claim 12, further comprising asecond chamber surrounding the other of the first piston rod portion orthe second piston rod portion.
 20. The engine of claim 12, wherein thepiston is configured to travel in a second stroke from the secondposition to the first position.
 21. The engine of claim 20, wherein thechamber is configured to have a substantially constant volume while thepiston moves in the first stroke and the second stroke.
 22. The engineof claim 12, wherein the first piston rod portion or the second pistonrod portion extends beyond the chamber.